1. Field of the Invention
The present invention relates to a shaft seal which is used in a rotary shaft and the like of a large-scale fluid machine, such as a gas turbine, a steam turbine, a compressor, a waterwheel, a refrigerator, and a pump. In particular, the present invention relates to a gas turbine which leads gas of high temperature and high pressure into a turbine, expands the gas therein, and generates dynamic force by converting thermal energy of the gas to mechanical rotation energy, and a shaft seal which is suitably used in a shaft comprising the gas turbine.
2. Description of the Related Art
In a gas turbine, a shaft seal for reducing the amount of gas which leaks from the high pressure side to the low pressure side is provided between stationary blades and the rotary shaft. As this shaft seal, a non-contacting labyrinth seal has been widely used. However, in this labyrinth seal, the gap between the tips of the fins must be increased by a certain amount so that the tips of the fins do not contact due to an axial vibration or an excessive thermal deformation during transient rotation. Consequently, a large amount of gas is leaked. Instead of this labyrinth seal, a brush seal has been developed with the aim of reducing the amount of leakage.
FIGS. 16A and 16B are schematic diagrams showing the constitution of this type of brush seal. In FIGS. 16A and 16B, reference numeral 1 represents a rotary shaft, reference numeral 2 represents a casing, reference numeral 3 represents a low pressure side side-plate, reference numeral 4 represents a high pressure side side-plate, reference numeral 5 represents a brazing section, and reference numeral 6 represents wires. The wires 6 is made of filaments which have a diameter of 50 to 100 μm and are flexible enough to be able to absorb decentering caused the vibration of the rotary shaft 1 or by thermal deformation. The wires 6 are bundled tightly into a bundle having a width of 3 to 5 mm so that there are no gaps between them. Furthermore, the wires 6 are attached at a gradient to the direction of rotation so as to form an acute angle with the outer periphery of the rotary shaft 1. The tips of the wires 6 touch the outer periphery of the rotary shaft 1 with a predetermined preliminary pressure, thereby reducing the amount of leakage in the axial direction.
The wires 6 slide against the rotary shaft 1 while contacting, generating heat dependent on atmospheric conditions or peripheral speed and becoming red-hot. For this reason, a material which is resistance to high temperatures, such as INCONEL alloy or HASTELLOY alloy, is used for the wires 6 in accordance with the conditions of use. Since the sliding face on the outer rim of the rotary shaft 1 is corroded together with the wires 6, it is coated with a corrosion-resistant material. Moreover, since the wires 6 have little flexibility in the axial direction of the rotary shaft 1, the wires 6 are prevented from being damaged by making the inside diameter of the low pressure side side-plate 3 substantially equal to the outer rim of the rotary shaft 1.
In addition, a leaf seal has been developed, such as that shown for example in FIG. 17. As shown in FIG. 17, the leaf seal 10 comprises flat thin plates 18 having a predetermined width, provided in multiple layers in the axial direction of the rotary shaft 11 at the rim thereof.
The outer base ends of the thin plates 18 are brazed inside a casing 12, that is, the thin plate 18 is attached to the casing by brazing section 15, the sealing the outer rim of the rotary shaft 11 and thereby dividing the peripheral space of the rotary shaft 11 into a high pressure region and a low pressure region. A high pressure side side-plate 14 and a low pressure side side-plate 13 are attached respectively in the high pressure region and the low pressure region at both sides of the thin plates 18, and function as guides in the direction of pressure.
The thin plates 18 have predetermined rigidity, which is determined by the thickness of the plates, in the axial direction of the rotary shaft 11. The thin plates 18 are attached to the casing 12 so that the angle they form with the rim of the rotary shaft 11 is acute with respect to the axial direction of the rotary shaft 11. Although the tips of the thin plates 18 contact the rotary shaft 11 with a predetermined pressure when the rotary shaft 11 is stationary, the dynamic pressure created by the rotation of the rotary shaft 11 causes the tips of the thin plates 18 to rise upward when the rotary shaft 11 is rotating, so that there is no contact between the thin plates 18 and the rotary shaft 11.
A slight gap 19 is provided between each of the laminated flat thin plates 18. Since the seal diameter is sufficiently large, i.e. since the diameter of the rotary shaft 11 is sufficiently large, each gap 19 can be regarded as approximately constant from the outer side of the rim to the inner side of the rim.
In the shaft seal having the constitution described above, the thin plates 18 having a width in the axial direction of the rotary shaft 11 are laminated in the peripheral direction of the rotary shaft 11. The thin plates 18 have gentle flexibility in the peripheral direction of the rotary shaft 11, and the seal mechanism has high rigidity in the axial direction of the rotary shaft 11.
In the above seal mechanism, since the thin plates 18 which are the seal member are provided in parallel in the axial direction of the rotary shaft 11, the outer side brazing which is affixed to the casing 12 is secure in the axial direction of the rotary shaft 11. Consequently, it is possible to prevent the thin plates 18 from becoming removed from the casing 12; this is a drawback of conventional brush seals, in which the wires may become removed from the casing.
However, the brush seal described above has the following problems.
There is a problem of leakage from between the wires 6, and from the sliding face on the outer rim of the rotary shaft 1 which touches the wires 6, but when the seal differential pressure exceeds a tolerance level which is determined by the diameter of the wires 6 and the arrangement of the low pressure side side-plates 3, and the like, all the wires 6 will become distorted toward the low pressure side and break, creating a blow-through space between the wires 6 and the rotary shaft 1 with the consequence loss of the seal function.
The rigidity of the wires 6 which comprise the brush seal is determined by their ability to follow the axial vibration of the rotary shaft 1, the appropriate pressure against the rotary shaft 1, and the like, and there is a limit on the extent to which flexibility can be increased by increasing the diameter of the wires 6 and the like. Therefore, the seal differential pressure in the axial direction of the rotary shaft 1, which is dependent on the flexibility of the wires 6, has a maximum of approximately 0.5 MPa, and no differential pressure larger than this can be sealed. Since the wires 6 are extremely thin, in general, they have a diameter of approximately 50 to 100 μm, there is a danger that the wires 6 will break and fall out as a result of sliding against the rim of the rotary shaft 1; this makes it difficult to use the gas turbine for a long period of time.
Since the wires 6 slide against the peripheral face of the rotary shaft 1, the amount of gas which leaks from the tips of the wires 6 is markedly less than in the labyrinth seal and the like. However, it is difficult to stabilize the leakage between the tips of the wires 6 at a low amount.
Since the wires 6 and the peripheral face of the rotary shaft 1 slide against each other, a corrosion-resistant must be provided on the surface of the rotary shaft 1. However, there is no established method of applying a corrosion-resistant coating, which is resistant over a prolonged period of use, to the peripheral face of a large-diameter rotary shaft. Since the wires 6 and the rotary shaft 1 are considerably corroded, the life of the brush seal is short and it must be frequently replaced.
Furthermore, the leaf seal 10 described above has the following problems.
The dynamic pressure generated by the rotation of the rotary shaft 11 causes the tips of the thin plates 18 to rise upward from the surface of the rotary shaft 11, so that there is no contact between the thin plates 18 and the rotary shaft 11. This constitution prevents excessive heat generation and corrosion. However, in the case where the low pressure side side-plate 13 and the high pressure side side-plate 14 are provided in such a manner that the gap between the low pressure side side-plate 13 and the thin plates 18 is substantially equal to the gap between the high pressure side side-plate 14 and the thin plates 18, when pressure has been applied from the high pressure side, the additional pressure load deforms the thin plates 18 toward the center of the radial direction of the rotary shaft 11, making it difficult to stop the thin plates from touching the rotary shaft 11 when the leaf seal is activated with small dynamic pressure and the like.
For these reasons, the brush seal and leaf seal described above require further improvements to reduce gas leakage and increase corrosion-resistance.